The invention relates to providing low-coherence light.
Sources of coherent light typically include an optical cavity that includes a gain medium. Gain media can include media that provide gain through “population inversion” such as a semiconductor gain medium of a solid-state laser, or other types of media such as a nonlinear optical medium that provides gain through nonlinear conversion.
Tunable sources of coherent light are applicable to a number of diagnostic and therapeutic medical applications. Optical coherence tomography is used to provide spatial resolution, enabling the imaging of internal structures. Spectroscopy is used to characterize the composition of structures, enabling the diagnosis of medical conditions, by differentiating between cancerous, dysplastic, and normal cellular structures.
In some cases the coherence of the light can lead to constructive and/or destructive interference over a transverse profile of a beam when the beam travels through a scattering medium (e.g., blood) to a sample. This “speckle effect” can lead to noise in a detected image or spectral signal.
Speckle can also be caused in other contexts including, for example, after a beam traveling through air scatters from a rough surface, such as in a laser projector illuminating a rough screen or wall.
Some approaches to reducing speckle in various contexts include reducing the temporal coherence of the light by broadening the linewidth of the source, splitting the illuminating wavefront into beamlets and delaying them relative to each other by longer than the coherence time of the source, or vibrating the sample to average out fluctuations.